1. Technical Field
This invention relates generally to laser beam modulation, and more particularly to such laser beam modulation used in laser printing/writing devices.
2. Background Art
Multiple level printing and writing systems that utilize sensitized media require that the media be exposed in a modulated fashion. This modulated exposure results in various image densities. A typical system may have the ability to generate image densities from 0.2 to 3.0. From data about the selected media (i.e. density vs. exposure) it can be determined that a range of densities will require a particular range of exposure energies. The dynamic range of these exposure energies may be defined as a ratio of the maximum exposure energy to the minimum exposure energy. A typical system utilizing silver halide film or sensitized paper may require a ratio of dynamic ranges of exposure energies greater than 100:1.
Exposure modulation in laser printing/writing devices may be accomplished through amplitude modulation and/or pulse-width modulation of the writing laser beam. One method currently in use is to modulate the beam of a gas (e.g. HeNe) or semiconductor (e.g. diode) laser with an external device such as an acousto-optic modulator. This method is fairly expensive, as it requires additional optic elements to propagate and shape the beam for modulation by such a device. Also, the modulating devices themselves are usually expensive.
To reduce the cost of a laser modulation system, it is desirable to directly modulate the output power of the laser source. This can be accomplished for example by modulating the drive current of a semiconductor laser diode. A limitation of direct modulation of a laser diode is that there is a threshold output power of the laser, above which the diode output is predominantly stimulated emission laser light and below which the diode output is predominantly spontaneous emission (FIG. 1). An output of predominantly spontaneous emission is undesirable as it does not propagate uniformly through the optical system to the exposure media. This limits the useful output power of the laser diode to those output powers generated above the threshold.
The ratio of the maximum output power of the laser diode to the output power of the laser diode available just above the threshold is considered the dynamic range of the device. In a typical laser diode this dynamic range is of the order of 15:1 to 50:1. This is below the requirements of many printing/writing systems.
Since most media are sensitive to the total integrated exposure, a pulse-width modulation approach, in addition to some amplitude modulation, may be used to achieve an improved dynamic range. An example of such a method is described in U.S. Pat. No. 4,774,710 which issued to Davis et al. in September, 1988. To implement pulse-width modulation at the desired pixel rates, though, higher speed circuitry is required (i.e. a fast settling digital-to-analog converter). This higher speed circuitry is required to maintain a particular throughput rate, as each pixel is now divided into many sub-pixels. For example, a 4-bit pulse-width modulation implementation writes sixteen subpixels, each of which are written in one-sixteenth of the time used to write a non-pulse width modulation pixel. This may require custom development of high speed devices from discrete components, since these circuits may not be commercially available as integrated devices.